Audio output device

ABSTRACT

Provided is an audio output device capable of preventing noises and improving the S/N ratio even if no audio signal is inputted in the middle of the input of audio signals, or even if an audio-signal input state and a no-signal state are alternately repeated. In the provided audio output device, a multiplier is provided on the input side of each of the delayers. Each multiplier multiplies the addition output of the corresponding one of adders by a multiplier coefficient supplied by the coefficient counter. If there is no input of digital audio signals into a ΔΣ modulator, the counter control circuit decreases the output of the coefficient counter down to 0 stepwise at predetermined intervals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an audio output device used in apparatuses tooutput sounds, such as TV sets, radio-cassette players, car audios, hometheaters, and component stereo sets.

2. Description of the Related Art

With recent development of LSI technologies, digital audio apparatuses,such as CD players and MD players, come to use 1-bit digital analogconverters (DACs) to process and amplify digital signals. The 1-bit DACperforms noise shaping on audio signals by using a ΔΣ modulator, andthen outputs the resultant audio signals as pulse-width modulatedsignals, specifically, as 1-bit PWM signals.

A conventional first-order ΔΣ modulator as one type of above-describedΔΣ modulator has a configuration shown in FIG. 4. FIG. 4 illustratescomponents of the first-order ΔΣ modulator by using a Z functionobtained through the Z-transform. Note that Z⁻¹ represents a delayelement to delay the input by one sampling clock.

The first-order ΔΣ modulator shown in FIG. 4 includes: a subtractor 81;an accumulator 90 that includes an adder 82 and a delayer 84; a delayer85; a multiplier 86; and a quantizer (Q) 83. The subtractor 81 subtractsa feedback signal W from an input signal X. The accumulator 90accumulates outputs S of the subtractor 81 for every sampling clock.

The quantizer 83 generates binary-quantized output signals Q(Z) byoutputting “+Δ” if the output Y of the accumulator 90 is equal to orlarger than zero (i.e., Y≧0), and by outputting “−Δ” if the output Y ofthe accumulator 90 is smaller than zero (i.e., Y<0). Each output signalQ(Z) is delayed by the delayer 85 by one sampling clock, and is inputtedas the feedback signal W to the subtractor 81 through the multiplier 86.

The first-order ΔΣ modulator is a feedback system with theabove-described components. The first-order ΔΣ modulator is a modulatorto convert the input signals X with a dynamic range from −Δ to +Δ intobinary-quantized signals (+Δ, −Δ) in synchronization with the samplingclocks for the two delayers 84 and 85.

Apparatuses to output sounds, such as TV sets and audio players by useof such ΔΣ modulators as one described above usually use higher-order ΔΣmodulators to improve the quality of the sounds. As described inJapanese Patent Application Publications Nos. Hei 9-307447, 2001-237707,and 2003-298425, a higher-order ΔΣ modulator includes pluralaccumulators and quantizers cascaded within a single loop.

SUMMARY OF THE INVENTION

Here suppose a case where a conventional ΔΣ modulator receives a digitalaudio signal, and then comes to receive no signal. In this case,provided with an accumulator using a delayer as described above, the ΔΣmodulator holds the accumulated data even if the ΔΣ modulator receivesno input. The remaining data circulate as a feedback signal in theaccumulator, and generate small noises to worsen the S/N ratio. If ahigher-order ΔΣ modulator is used, the plural cascaded accumulatorsincrease the influence of the noises.

The invention has been made to solve the above-described problem, and anobject of the invention is to provide an audio output device capable ofpreventing noises and improving the S/N ratio even if no audio signal isinputted in the middle of the input of audio signals, or even if anaudio-signal input state and a no-signal state are alternately repeated.

To achieve the above-described object, an audio output device accordingto the invention is mainly characterized by comprising a ΔΣ modulatorincluding: a quantizer configured to quantize a signal; a subtractorconfigured to subtract a feedback signal from the quantizer from aninputted digital audio signal; an accumulator configured to accumulateoutput signals from the subtractor and to output the accumulated outputsignals to the quantizer side; a delayer included in the accumulator; amultiplier included in the accumulator and connected to an input side ofthe delayer; and a multiplier coefficient control circuit configured tosupply a multiplier coefficient to the multiplier, wherein, in the ΔΣmodulator, the multiplier coefficient control circuit decreases themultiplier coefficient down to 0 stepwise at predetermined intervals ifthere is no input of the digital audio signal.

The audio output device of the invention includes a ΔΣ modulator thatincludes an accumulator including both a delayer and a multiplierprovided on the input side of the delayer. The multiplier coefficient ofthe multiplier is controlled by a multiplier coefficient controlcircuit. If there is no input of digital audio signals, the ΔΣ modulatormakes the multiplier coefficient control circuit decrease the multipliercoefficient down to 0 stepwise at predetermined intervals. Accordingly,the audio output device can delete the data that remain in theaccumulator, and thus can prevent the noises that would otherwise begenerated if there is no input of signals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram illustrating the configuration of anaudio output device according to the invention.

FIG. 2 is a block diagram illustrating the configuration of a ΔΣmodulator provided in the audio output device according to theinvention.

FIG. 3 is a chart illustrating the relationship between thepre-processing input signal in the ΔΣ modulator and the coefficient setin a multiplier provided in an accumulator.

FIG. 4 is a block diagram illustrating the configuration of aconventional first-order ΔΣ modulator.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the invention is described below by referring to thedrawings. The drawings are schematic, and are different from thereality. Moreover, the drawings also include portions having differentdimensional relationships and ratios from each other.

FIG. 1 illustrates a basic configuration of an audio output device 10according to the invention. The audio output device 10 includes asynchronized sampling rate converter 1, a digital signal processor (DSP)2, an over-sampling filter 3, a ΔΣ modulator 4, a PWM modulator 5, andthe like.

The synchronized sampling rate converter 1 converts sampling frequenciesof 8 kHz, 12 kHz, 16 kHz, 24 kHz, 32 kHz, 48 kHz, 96 kHz, and the likeof PCM signals, which are inputted digital audio signals, into asampling frequency of 48 kHz that is suitable for processing at laterstages. In addition, the synchronized sampling rate converter 1 convertssampling frequencies of 11.025 kHz, 22.05 kHz, 44.1 kHz, 88.2 kHz, andthe like of the inputted PCM signals into a sampling frequency of 44.1kHz that is suitable for processing at the later stages.

The signals outputted from the synchronized sampling rate converter 1are inputted into the DSP 2, and are configured by, for instance, aparametric-equalizer circuit or the like, where adjustment or the likeare performed on the volume and the frequency characteristics of thedigital audio signals.

After the signal processing performed in the DSP 2, the data areinputted into the over-sampling filter 3. The over-sampling filter 3performs an over-sampling based on the input sampling frequency of thesignals inputted into the over-sampling filter 3. The over-sampling isperformed at such frequencies as the double, the quadruple, or theoctuple of the input sampling frequency.

The ΔΣ modulator 4 modulates the PCM signals inputted from theover-sampling filter 3 into multi-value PDM (pulse density modulation)signals. The PWM modulator 5 modulates the inputted PDM signals into PWMsignals for each of the L/R channels, that is, into 1-bit (binary)signals. Here, the ΔΣ modulator 4 may be configured to modulate the PCMsignals into binary PDM signals in advance.

The processing of signals described thus far is a pure, digital signalprocessing. The operations of the processing are controlled on the basisof the reference clock signals generated by, for instance, anunillustrated crystal oscillator circuit.

Subsequently, for instance, the PWM-outputted signals for each of theL/R channels are subjected to a switching amplification, and then, fromthe amplified signals, unnecessary high-frequency content (noisecontent) is removed by a LPF or the like. The resultant signals are sentto a speaker or the like.

The ΔΣ modulator 4 includes, for instance, a fifth-order ΔΣ modulatorincluding five accumulators as shown in FIG. 2. Detail description ofthe ΔΣ modulator 4 is provided below. In the ΔΣ modulator shown in FIG.2, within a single loop starting from the pre-processed digital audiosignal serving as the signal for an input A and ending with the feedbackfrom the quantizer 58 to the input A side, five accumulators 71 to 75are cascaded between the input terminal of the input A and the quantizer58. The output of the quantizer 58 is fed back to the accumulators 71 to75 via multipliers 25, 31, 39, 45, and 53 respectively.

The quantizer 58 performs a quantization processing on the output of theaccumulator 75 of the fifth stage, and thus derives quantized data.

The accumulator 71 of the first stage makes an addition output of anadder 26 pass through a multiplier 27, and then delays the resultantdata by means of a delayer 28. Then, the accumulator 71 returns thedelayed data back to the adder 26 through a feedback loop, and adds thereturned data to the subtraction output of a subtractor 24. Accordingly,the accumulator 71 accumulates the subtraction outputs of the subtractor24 for every sampling clock, that is, performs an integral treatment.The integral treatment function is similarly performed by the otheraccumulators 72 to 75. Accordingly, no further description of theaccumulation function will be provided below.

The quantized data from the quantizer 58 are inputted into themultiplier 25, and then the multiplication output of the multiplier 25is fed back to the subtractor 24. The subtractor 24 subtracts themultiplication output of the multiplier 25 from the multiplicationoutput of a multiplier 23, where the digital audio signal from the inputA is multiplied.

The accumulator 72 of the second stage returns the addition output of anadder 33 back to a feedback loop. In the feedback loop, the additionoutput is made to pass through a multiplier 35 and is then delayed by adelayer 34. The resultant data are returned back to the adder 33, wherethe returned data are added to the addition output of an adder 32.

The quantized data fed back from the quantizer 58 are multiplied by themultiplier 31. A subtractor 30 is provided to subtract themultiplication output of the multiplier 31 from the multiplicationoutput of a multiplier 29. The subtraction output of the subtractor 30is added by the adder 32, provided on the input side of the accumulator72 of the second stage, to the output of the multiplication performed bya multiplier 36 on the output of the accumulator 73 of the third stage.

The accumulator 73 of the third stage makes the addition output of anadder 40 pass through a multiplier 41. The resultant data are delayed bya delayer 42, and are then returned back to the adder 40 through afeedback loop. The fed-back data are added to the subtraction output ofa subtractor 38.

The quantized data fed back from the quantizer 58 are multiplied by themultiplier 39. The subtractor 38 is provided to subtract themultiplication output of the multiplier 39 from the multiplicationoutput of a multiplier 37.

The accumulator 74 of the fourth stage returns the addition output of anadder 47 back to a feedback loop. In the feedback loop, the additionoutput is made to pass through a multiplier 49 and is then delayed by adelayer 48. The resultant data are returned back to the adder 47, wherethe returned data are added to the addition output of an adder 46.

The quantized data fed back from the quantizer 58 are multiplied by themultiplier 45. A subtractor 44 is provided to subtract themultiplication output of the multiplier 45 from the multiplicationoutput of a multiplier 43. The subtraction output of the subtractor 44is added by the adder 46, provided on the input side of the accumulator74 of the fourth stage, to the output of the multiplication performed bya multiplier 50 on the output of the accumulator 75 of the fifth stage.

The accumulator 75 of the fifth stage makes the addition output of anadder 54 pass through a multiplier 55. The resultant data are delayed bya delayer 56, and are then returned back to the adder 54 through afeedback loop. The fed-back data are added to the subtraction output ofa subtractor 52.

The quantized data fed back from the quantizer 58 are multiplied by themultiplier 53. The subtractor 52 is provided to subtract themultiplication output of the multiplier 53 from the multiplicationoutput of a multiplier 51.

As has been described above, the multiplier 29 is provided on the outputside of the accumulator 71. The multiplier 37 is provided on the outputside of the accumulator 72. The multiplier 43 is provided on the outputside of the accumulator 73. The multiplier 51 is provided on the outputside of the accumulator 74. In addition, a multiplier 57 is provided onthe output side of the accumulator 75. These multipliers 29, 37, 43, 51,and 57 serve as attenuators. In the higher-order ΔΣ modulator, each ofthe multiplier 29, 37, 43, 51, and 57 provided to prevent oscillationhas a multiplier coefficient smaller than one.

In the accumulators 71 to 75, the multipliers 27, 35, 41, 49, and 55 arerespectively provided on the input sides of their corresponding delayers28, 34, 42, 48, and 56, and are respectively connected to theircorresponding delayers 28, 34, 42, 48, and 56. Each of the multipliers27, 35, 41, 49, and 55 multiplies the addition output of thecorresponding one of the adders 26, 33, 40, 47, and 54 by a multipliercoefficient supplied from a coefficient counter 22.

An input level detector circuit 20 is connected to a terminal of theinput A. The input level detector circuit 20 is a circuit to detectwhether or not there is any digital audio signal being inputted into theinput A. In the detection, a threshold of a certain level ispredetermined to distinguish digital audio signals from noise content.If the input level that is not over the threshold continues for acertain length of time, the input level detector circuit 20 judges thatthere is no digital audio signal being inputted. (i.e., input of digitalaudio signals=0). If detecting a digital signal exceeding the thresholdafter a judgment of “input of digital audio signals=0,” the input leveldetector circuit 20 judges that another digital audio signal is beinginputted.

The output of the input level detector circuit 20 is supplied to amultiplier coefficient control circuit 60. The multiplier coefficientcontrol circuit 60 includes a counter control circuit 21 and thecoefficient counter 22. The multiplier coefficient control circuit 60controls the multiplier coefficients to be supplied to each of themultipliers 27, 35, 41, 49, and 55. Each of the multiplier coefficientsbasically has a value ranging from 0 to 1.

The counter control circuit 21 controls the coefficient counter 22.Specifically, the counter control circuit 21 gives a preset value to thecoefficient counter 22 serving as a counter. In addition, the countercontrol circuit 21 makes the coefficient counter 22 perform an adding orsubtracting operation on the count. In the initial state, the countercontrol circuit 21 gives a value 1 to the coefficient counter 22 as itspreset value.

If a judgment signal indicating that “the digital-audio-signal input=0”is supplied by the input level detector circuit 20 to the countercontrol circuit 21, the counter control circuit 21 controls thecoefficient counter 22 so that the count of the coefficient counter 22can be reduced gradually from 1 to 0. If a judgment signal indicatingthat “the digital-audio-signal input=0” supplied to the counter controlcircuit 21 is followed by the reception of a judgment signal indicatingthe input of another digital audio signal, the counter control circuit21 controls the coefficient counter 22 so that the count of thecoefficient counter 22 can be increased gradually from 0 to 1.

Basic operations of the input level detector circuit 20 and themultiplier coefficient control circuit 60 are described below byreferring to FIG. 3. The upper side of FIG. 3 is dedicated to thesignals supplied to the input A while the lower side is dedicated to theoutput of multiplier coefficients from the coefficient counter 22.

Digital audio signals that are the output of the over-sampling filter 3shown in FIG. 1 are inputted into the input A. The digital audio signalsare supplied through the multiplier 23 to the subtractor 24, where thefeedback signal outputted by the quantizer 58 is subtracted from themultiplication output of the multiplier 23. The output of the subtractor24 is supplied to the accumulator 71 of the first stage. From thenonwards, the signals are processed in the course of the configurationdescribed above. What characterizes the signal processing is themultiplier provided on the input side of the delayer in each of theaccumulators and the use of the output of the multiplier as the input ofthe corresponding delayer.

FIG. 3 shows that, in the initial state, digital audio signals arecontinuously inputted by the over-sampling filter 3 to the ΔΣ modulator4. In the meanwhile, a multiplier coefficient of 1 is supplied to eachof the multipliers 27, 35, 41, 49, and 55.

Then, at time t0, there is no digital audio signal being supplied to theinput A (i.e., input signal=0). At this point of time, the input leveldetector circuit 20 detects the fact that there is no input signal(i.e., input signal=0) by comparing the level of the signal with apredetermined threshold, and judges whether or not the detected state of“input signal=0” continues uninterruptedly for a predetermined length oftime—e.g., for 43 ms. At time t1, which is 43 ms after time t0, theinput level detector circuit 20 judges that there is no input of digitalaudio signals (e.g., input of digital audio signals=0), and sends thesignal indicating the judgment result to the counter control circuit 21.

Upon receiving the judgment signal indicating “input of digital audiosignals=0” from the input level detector circuit 20, the counter controlcircuit 21 performs a count-down control on the coefficient counter 22.How the counting down is going is shown in the period from time t1 totime t2. In the period from time t1 to t2, the coefficient counter 22transitions from the state of 1 to the state of 0 stepwise. Thetransition time from time t1 to time t2 is, for instance, 20 ms.

Once the coefficient counter 22 comes to have a value 0, a multipliercoefficient 0 is supplied (set) to each of the multipliers 27, 35, 41,49, and 55 provided respectively in the accumulators 71 to 75. Hence,the multiplier 27, for instance, outputs a value obtained by multiplyingthe addition output of the adder 26 by the multiplier coefficient 0.Likewise, each of the other multipliers 35, 41, 49, and 55 outputs avalue obtained by multiplying the addition output of the correspondingone of the adders 33, 40, 47, and 54 by multiplier coefficient 0. Inthis way, each of the delayers 28, 34, 42, 48, and 56 providedrespectively in the accumulators 71 to 75 has an input of 0, and thusthe data that would otherwise cause noises disappear.

At time t3, the input A that has been in a state of “input of digitalaudio signals=0” comes to receive a new input of digital audio signals.At this point of time, the input level detector circuit 20 detects theexistence of digital audio signals, and sends the detection signal tothe counter control circuit 21. Upon receiving the judgment signalindicating the new input of digital audio signals, the counter controlcircuit 21 performs a count-up control on the coefficient counter 22.Then, as shown in the period from time t3 to time t4, the coefficientcounter 22 transitions from the state of 0 to the state of 1 stepwise.The transition time from time t1 to t2 is, for instance, 5 ms.

As has been described thus far, even if the input of digital audiosignals is interrupted to leave the input side of the accumulator withno input signal, and the data remaining in the delayer provided in theaccumulator are fed back to the delayer through a feedback loop, themultiplier makes the delayer eventually have an input of 0. Accordingly,the noises that would otherwise be caused by the remaining data can beeliminated.

Note that a fifth-order ΔΣ modulator is used in the example describedabove, but a fourth-order, a six-order, or a seventh-order ΔΣ modulatormay be used instead. Alternatively, a lower-order ΔΣ modulator, such asa second-order or a first-order ΔΣ modulator may be used instead. Inaddition, the ΔΣ modulator and the input/output signals may be of pluralbits instead of those of 1-bit.

The configuration of the audio output device according to the inventionis widely applicable not only to TV sets, radio-cassette players, caraudios, home theaters, and component stereo sets but also to any systemto perform transmission by sounds.

What is claimed is:
 1. An audio output device comprising a ΔΣ modulatorincluding: a quantizer configured to quantize a signal; a subtractorconfigured to subtract a feedback signal from the quantizer from aninputted digital audio signal; an accumulator configured to accumulateoutput signals from the subtractor and to output the accumulated outputsignals to the quantizer side; a delayer included in the accumulator; amultiplier included in the accumulator and connected to an input side ofthe delayer; and a multiplier coefficient control circuit configured tosupply a multiplier coefficient to the multiplier, wherein in the ΔΣmodulator, the multiplier coefficient control circuit decreases themultiplier coefficient down to 0 stepwise at predetermined intervals ifthere is no input of the digital audio signal.
 2. The audio outputdevice according to claim 1, wherein a plurality of the accumulators arecascaded between an input terminal for digital audio signals and thequantizer, the feedback signal from the quantizer is fed back to each ofthe plurality of accumulators, and the multiplier coefficient of themultiplier coefficient control circuit is supplied to each of themultipliers provided respectively in the plurality of accumulators. 3.The audio output device according to claim 1, wherein if an input of thedigital audio signal restarts, the multiplier coefficient controlcircuit increases the multiplier coefficient up to 1 stepwise atpredetermined intervals.
 4. The audio output device according to claim1, further comprising an input level detector circuit judging thepresence of the digital audio signal, wherein the input level detectorcircuit judges that there is no digital audio signal if the input levelof the digital audio signal that is not over a predetermined thresholdcontinues for a certain length of time.
 5. The audio output deviceaccording to claim 4, wherein the multiplier coefficient control circuitincludes a counter control circuit and a coefficient counter, and ajudgment signal by the input level detector circuit is supplied to thecounter control circuit.
 6. The audio output device according to claim5, wherein the counter control circuit performs a count-down control onthe coefficient counter by the judgment signal indicating no digitalaudio signal from the input level detector circuit.
 7. The audio outputdevice according to claim 1, wherein the accumulator includes thedelayer and the multiplier and an adder.
 8. The audio output deviceaccording to claim 2, further comprising a multiplier for an attenuationwherein the multiplier for an attenuation is connected between the eachaccumulator.